However, for one box I'm working on, this is only possible if the subscriber's address is hosted on the same server as the list being joined. If one tries to subscribe in this way from gmail or another service, the cPanel server responds with:

Code:

SMTP error from remote mail server after RCPT TO:<foo-join@domain.com>:
host aspmx.l.google.com [74.125.47.26]: 550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or